This invention relates to methods and apparatus for utilizing solar energy in systems which have solar collector assemblies with absorber panels through which a heat-receiving liquid is circulated. Systems of this general type are well-known, but are subject to certain disadvantages which have been recognized by persons working in this field.
Most absorber panels are expensive since they must be made of a material which has good thermal conductivity and possesses sufficient strength to hold pressurized liquid.
When prior absorber panels have been made of materials of sufficient strength to withstand normal system operating pressures and to provide sufficient thermal conductance, the thickness of the material adds an expense to the system and adds to the thermal capacity and warmup time.
Some prior absorber panels have been constructed of sheets which are bonded together at spaced-apart locations, to provide a flow passage formed of laterally spaced segments. Such an arrangement is undesirable in several respects. First, the heat from solar energy which strikes the areas of the absorber panels between the flow passage segments must be conducted laterally in the panel to the heat exchange liquid in the flow passage.
Portions of the absorber panel are at temperatures which are higher than the temperature of the heat-exchange fluid, resulting in undesired losses of heat from the collector assembly. A further disadvantage of this prior construction is that the bonding together of the adjacent sheets leaves them susceptible to damage from distension or rupture when there is excessive flow through the absorber panel or when the flow passage is expanded due to freezing of the liquid therein.
Many prior systems provide flow passages which are filled by a layer of air and a layer of water or other heat exchange liquid. In these systems, the optimum transfer of heat to the heat exchange liquid is not possible since the upper sheet is insulated from the liquid by the air which is present in the flow passage. If the top layer of the passage is transparent, condensation of water vapor occurs on the passage surface thus reducing its transmittance.
The present invention includes a number of concepts specifically set forth in the claims below. These concepts when combined and sometimes when taken alone, avoid one or more of the aforementioned shortcomings of the prior art.
One feature of the present invention is the presence of a minimum pressure differential across the sheets which form the absorber panel. Preferably, this is achieved by the utilization of a circulatory system which causes the heat exchange fluid to flow through the flow passage in the absorber panel at a sub-atmospheric pressure. Such a circulatory system may involve an evacuated reservoir located above the inlet end of the absorber panel, a reservoir connected to the absorber panel by means such as an orifice or pipe of dimensions to produce a pressure drop resulting in negative pressure, or a reservoir at atmospheric pressure which is in communication with and at a lower elevation than the inlet end of the absorber panel.
Another feature of the invention is the use in a liquid-filled system of spacer means between the sheets which form the absorber panels, the spacer means serving to maintain a spaced relationship between the sheets when the pressure differential across the sheets tends to bias them toward each other. When the spacers are small, the flow through the absorber panel is substantially equivalent to flow between parallel plates, avoiding any significant lateral thermal conduction in the absorber panel. When the sheets are unbonded at the spacer means, they are free to move apart to avoid permanent damage to the absorber panel in the event the sheets are forced apart by thermal expansion or system malfunction.
Many of the objects of this invention are realized as a direct result of one or more of the features described above. One such object is to permit the use of economical absorber panels made of relatively thin sheets of material. This represents a substantial saving, particularly in absorber panels which are made of copper or other corrosion-resistant materials.
Another object of the invention is to provide an uncomplicated and relatively maintenance-free circulatory system.
Still another object of the invention is to provide a system which is highly efficient to provide maximum utilization of the solar energy. Many features of the invention contribute to this. According to most preferred embodiments, the flow passage extends substantially across the total area of the absorber panel. The use of thin materials and the avoidance of substantial lateral separation between the portions of the flow passage causes all parts of the sheet exposed to solar energy to remain at a temperature approximately equal to that of the heat exchange fluid, thereby reducing the loss of heat due to radiation and convection from the absorber panel. The efficiency is also improved by having the single-phase flow of liquid which excludes air or other gases from the flow passage.
A further object of the invention is to provide a durable system. The absence of a positive connection between the sheets at the spacer means renders the absorber panel resistant to freeze damage.